From Wikipedia: 

“The acre is a unit of land area used in the imperial and US customary systems. It is defined as the area of 1 chain by 1 furlong (66 by 660 feet), which is exactly equal to ​1640 of a square mile, 43,560 square feet, approximately 4,047 m2, or about 40% of a hectare.”

An American football field is 360 x 160 feet or 57,600 square feet so an acre is less than that. It’s just under the 45,000 square feet of the 100 yard playing field.

So, basically.. “How big is an acre?”

Farmer “About as big as a Football field… but not really.”